The invention resides in a method for detecting the opening of a passive pressure limiting valve for the release of fuel from a common rail fuel injection system to a fuel tank wherein, during normal operation, a load reduction is recognized when the rail pressure exceeds a limit value and, as a result, a PWM (Pulse Width Modulation) signal is generated for temporarily applying to a suction throttle a control signal with a PWM value which is increased over the value provided during normal operation.
In a common rail fuel injection system, a high pressure pump supplies pressurized fuel to a rail. The fuel supply line cross-section to the high pressure pump is controlled by a variable suction throttle. The rail is in communication with fuel injectors by way of which the fuel is injected into the combustion chambers of the internal combustion engine. Since the quality of the combustion depends to a large extent on the pressure level in the rail, the pressure level is controlled. The high-pressure control circuit comprises a pressure controller, the suction throttle with high pressure pump and the rail and also a back coupling branch including a filter. In this high pressure control circuit, the pressure level in the rail corresponds to the control value. The pressure values as measured in the rail are converted via the filter to an actual rail pressure and compared with a desired rail pressure. The control deviation obtained thereby is converted via the pressure controller to a control signal of the suction throttle. The control signal corresponds for example to a volume flow with the unit liter/minute. Typically, the control signal is in the form of an electrical PWM signal (pulse-width modulated). The high pressure control circuit described above is known from DE 103 30 466 B3.
For protection from an excessive pressure level, the rail is provided with a passive pressure limiting valve. When the pressure level exceeds a predetermined value, the pressure limiting valve opens whereby fuel is released from the rail and returned to the fuel tank.
In practice, the following problem can occur: During a load reduction the engine speed increases momentarily. An increasing engine speed causes, with a constant desired engine speed, an increased speed control deviation. This causes a speed controller to reduce the control value for the fuel injection amount. A lower fuel injection amount again results in less fuel being taken out of the rail whereby the fuel pressure level in the rail is rapidly increased. Complicating the process is the fact that the pumping volume of the high pressure is speed dependent. An increase in the engine speed results in an increased pumping volume and, consequently, additionally increases the pressure in the rail. Since the high-pressure control has a comparatively long reaction time, the rail pressure can increase to such an extent that the pressure limiting valve opens for example at 1950 bar. As a result, the rail pressure drops for example to a value of 800 bar. At this pressure level, an equilibrium state between the pumped fuel and the released fuel amount is established so that the rail pressure does not drop any further. As a result of the pressure loss, the efficiency of the internal combustion engine drops while, at the same time, the exhaust gas become visibly murky.
The not-pre-published German patent application DE 10 2006 029 138.4 proposes a method wherein, upon recognizing a load reduction, the rail pressure is controlled by setting the PWM signal temporarily to a value which is higher than it would be under normal operation. By increasing the power supply energy to the suction throttle, the dynamics of the control member are increased, whereby an unintentional opening of the pressure limiting valve is suppressed while a suction throttle slide which is hard to move can nevertheless be operated.
However, with failures such as a cable breakage or a suction throttle plug not being correctly locked in position or a suction throttle slide member permanently locked up this method is ineffective so that the pressure limiting valve opens unexpectedly when the rail pressure increases.
It is therefore the object of the present invention to safely recognize an undesired opening of the passive pressure limiting valve of a common rail fuel injection system.